Accumulation and distribution characteristics of Cd in roots of cadmium-safe rice line at tillering stage

Objectives Cultivation of low Cd accumulation cultivars is an effective measure to minimize Cd influx into the human food chain. The mechanism of Cd immobilization in the roots of the cadmium-safe rice lines were studied in this paper.

Methods A hydroponic experiment was conducted using a cadmium-safe rice line (D62B) and a common rice line (Luhuil7) as the comparing material. The rice seedlings of 3-leaf-1-sprout stage were cultured in nutrient solution for a week, then exposed to solution of Cd at 0 (CK), 0.5 (Cd 0.5), 1 (Cd 1) and 2 (Cd 2) mg/L supplying with CdCl₂·2.5H₂O for 30 days. The plants were harvested and divided into roots and shoots to measure the Cd content, and the chemical forms and subcellular distribution of Cd in roots were analyzed, and the modifications of cell wall polysaccharides to Cd of roots cell wall were discussed.

Results Compared with Luhui17, D62B showed a lower Cd content in different parts, smaller transfer factor, and less Cd translocation from roots to shoots. Besides the decrease of the proportion of Cd extracted by d-H₂O in roots of two rice lines, the proportion of Cd extracted by 0.6 mg/L HCl and Cd in the residue exhibited an increasing trend with increasing Cd concentration in the solution, indicating that the mobility and activity of Cd in roots of two rice lines decreased with Cd treatments. Among different chemical forms of Cd in roots of D62B, the Cd extracted by 1 mol/L NaCl accounted for the largest part (48.9%−52.1%). Furthermore, its proportion was 1.11 times of that of Luhui17, while those extracted by d-H₂O were lower (82.3%) at 2.0 mg/L Cd treatment, indicating that the mobility and activity of Cd in roots of D62B were much lower than those of Luhui17. The vast majority of Cd was in soluble fraction and in the cell wall of the roots. The Cd in the roots cell wall of D62B was 38.6%−41.8%, higher than that of Luhui17. With increasing Cd content, the proportion of Cd in roots cell wall decreased, but D62B showed a greater capacity than Luhui17 to attain Cd within cell wall with a limit. As the hemicellulose 1 is the major site for Cd storage in roots cell wall, the Cd content in hemicellulose 1 of roots cell wall was 7.74−8.40 times that in pectin of D62B. The Cd content in hemicellulose 1 of two rice lines significantly increased with increasing Cd concentrations. The amount of Cd in polysaccharide of hemicellulose 1 of D62B and Luhui17 increased by 32.6% and 11.2%, respectively, at 2.0 mg/L Cd treatment compared to those at 1.0 mg/L Cd treatment. Furthermore, the total polysaccharide content in hemicellulose 1 as well as the Cd content of roots cell wall increased with the increase of Cd concentration, suggesting that the increase in hemicellulose 1 contributed greatly to the fixation of Cd²⁺ in the cell wall.

Conclusions The cadmium-safe rice line designated D62B showed lower Cd content in plant and smaller transfer factor than those of Luhui17. Cd extracted by 1 mol/L NaCl was the major form in roots of D62B. Increasing Cd concentration resulted in conversion of Cd to less mobile forms. Cd in roots cell wall was mainly fixed by hemicellulose 1, and D62B showed a stronger fixation than Luhui17. The stronger capacity of cell wall of D62B to retain Cd was associated with fixation of hemicellulose 1. Cd in the immobile form and immobilization in hemicellulose 1 of roots cell wall of cadmium-safe rice line restrains the translocation of Cd from roots to shoots, which is a major mechanism that differentiate the rice lines the in governing the accumulation of Cd in grains.